The present invention relates to an automatic adjusting filter and, more particularly, to a cut-off frequency automatic adjusting filter which has a reference filter separate from a filter actually used and to which a feedback is applied.
In a conventional automatic adjusting filter to which the present invention relates, a sine wave signal is inputted to an input terminal of a reference low-pass filter (hereinafter referred to as "reference LPF") as a reference signal so that the phase differences of the input signal and an output signal therefrom are compared by a phase difference comparator. However, in an ordinary electric circuit, generally it is rare that there exist such sine waves having an appropriate frequency for the purpose. Therefore, it has been necessary for such conventional circuit to be equipped with a sine wave generating circuit exclusively for the automatic adjusting filter. This presents numerous restrictions to the circuit designs, the system designs and the substrate designs in electric circuits. The detail arrangements of the typical conventional automatic adjusting filter and the problems existing therein will be described later with reference to the drawings.